Currently there are two principal types of power inverters. The first and simplest is called the “modified sine wave” or “square wave” inverter. With this topology, a 60 Hz square wave is used to drive the load directly, duty cycle adjusted to provide a desired RMS voltage (e.g., 120V RMS). This can be efficient since switching losses are almost nonexistent, with the devices being switched at the “low” line frequency. The second type of power inverter falls into two categories, filtered pulse width modulated (PWM) and non-filtered PWM. Filtered PWM is most applicable to single phase motor loads as the voltage is nearly a perfect sine wave. The problem with this approach is generally in the output filter, which often introduces an impracticably high source impedance that can cause instability on some switching power supplies. Unfiltered PWM has the problem of the PWM carrier being dissipated as heat in the motor's start capacitor. Since the PWM carrier is usually at least 10 KHz, it becomes a delicate balance to find a large enough start capacitor, but not one so large that its capacitive reactance causes significant power loss. Also, since the capacitor is almost always provided with the motor for optimal starting efficiency, the user typically does not provide input as to its value.
Optimally, a modified sine wave inverter is employed at low frequencies, for example, to start up a motor, and an unfiltered PWM inverter is used at higher frequencies. As noted, non-filtered PWM inverters are not well suited for single phase motor loads during startup and shutdown. This is because the motors typically have a rather large integral start capacitor shunted to the stator windings to facilitate the startup of the motor. This capacitor is removed from the motor load by a centrifugal switch after the motor has reached a sufficient speed. With a PWM motor drive, and before the start capacitor is switched out, the capacitive element tends to appear as a reactive component that shorts with the high frequency PWM carrier. This can cause the motor to overheat and/or degrade the life of the start capacitor.
An alternative approach would be to drive the single phase motor with only a square wave drive signal, however, problems associated with square wave drive signals are well know, with such signals generally not working very well at typical motor operating speeds. However, at low speeds, e.g., during startup or shutdown, a square wave drive signal (or quasi square wave drive signal) may be preferred.
Thus, a need remains in the art for an enhanced technique for generating a motor drive signal for a single phase AC motor, particularly during startup and shutdown transitioning of the motor which would ensure that a pulse width modulated drive signal is not applied to the motor before the centrifugal switch has disconnected the capacitor from the motor circuit.